I. Field of the Invention
This invention relates generally to an off-road, four-wheeled recreational vehicle, and more specifically to a vehicle of the type having its front and rear wheels equipped with wide, ultra-low pressure tires.
II. Discussion of the Prior Art
There has been proposed in the prior art (as in Japanese Patent Laid-Open No. 59-167333) the riding-type, off-road four-wheel drive vehicle which has its two front wheels and two rear wheels surrounded with wide and ulta-low pressure tires (i.e., the so-called "balloon tires"). Vehicles of this type generally have their body size made smaller than that of an ordinary automobile. As a result, the front tread width (i.e., the center-to-center distance of the two front wheels) is relatively small, while the tires to be attached to the wheels are relatively wide, and thence, little space is left between the two front steerable wheels to accommodate the drive and suspension systems.
Where the front wheels of the four-wheel drive vehicle are to be coupled to the frame by means of an independent suspension, a differential gear is usually attached to the vehicle body in the space between the two front wheels and driven shafts are coupled by universal joints (U-joints) to the differential output shafts and to the axles of the front wheels.
In vehicles having the construction described above, the driven shafts are necessarily short to fit between the wide tires and the differential and, hence, the U-joints of the driven shafts are subjected to significant bending forces when the front wheels move up and down or are steered due to the large pivot angle of the front wheel axles. As a result, the U-joints tend to become weakened due to the repeated bending moments. The prior art solution to this problem was to make the vertical stroke of the front wheels (i.e., the up and down wheel travel) small. If the vehicle is equipped with a front wheel independent suspension system, this results in degraded performance as far as comfort and ridability are concerned.
Furthermore, in the prior art design being discussed, when the vehicle is traveling uphill, the load distribution to the front wheels is reduced, as is the road gripping force of the front wheels. Hence, the front wheels tend to slip which makes it difficult to effectively transmit their drive force to the road surface.